This invention relates to a ferritic chromium alloyed steel formed from a melt deoxidized with titanium and having an as-cast fine equiaxed grain structure. More particularly, this invention relates to a ferritic chromium alloyed steel formed from a melt deoxidized with titanium and containing low aluminum. A hot processed sheet produced from the steel having this equiaxed grain microstructure is especially suitable for a cold reduced, recrystallization annealed sheet having excellent formability, stretching and non-ridging characteristics.
A requirement of a highly formable ferritic stainless steel, in addition to having a high r.sub.m, is that it be free of a phenomenon known as "ridging", "roping" or "ribbing". Unsightly ridging may appear on the surfaces of a cold reduced, recrystallization annealed ferritic stainless steel sheet that is to be subjected to cold forming. "Ridging" is characterized by the formation of ridges, grooves or corrugations which extend in a parallel direction to the rolling direction of the sheet. This defect not only is detrimental to the surface appearance of the sheet but also results in poor formability.
Ferritic chromium alloyed steels, especially sub-equilibrium chromium alloyed steels such as stainless Type 409, 430 and 439, typically have an as-cast columnar large grain structure, whether continuously cast into slab thicknesses of 50-200 mm, or strip cast into thicknesses of 2-10 mm. These columnar grains have a near cube-on-face crystallographic texture which leads to a very undesirable ridging characteristic in a final cold rolled, annealed sheet used in various fabricating applications. The surface appearance resulting from ridging is highly objectionable in exposed formed parts such as caskets, automotive trim, exhaust tubes and end cones, stamped mufflers, oil filters, and the like. Ridging causes the sheet to have a rough, uneven surface appearance after forming attributed to a cold rolled, annealed, large non-uniform grain size resulting from the initial occurrence of a columnar grain structure in the as-cast steel. This uneven surface appearance is aesthetically objectionable. To minimize ridging, an extra costly production step of annealing a hot rolled sheet prior to cold reduction is required. This extra annealing of the ferritic stainless steel also results in reduced formability caused by lower average strain ratios required for deep drawability. Additionally, a hot processed sheet that is annealed before cold reduction must be cold reduced at least 70% to obtain an r.sub.m value after final annealing similar to the r.sub.m value for a hot processed sheet that otherwise is not annealed before cold reduction.
Over the years, there also have been numerous attempts to eliminate ridging by modifying the alloy composition of ferritic stainless steel. It is known ridging in a ferritic stainless steel originates primarily during hot rolling. For example, there have been attempts to minimize ridging by casting a steel ingot by forming a fine equiaxed grain microstructure by controlling chemistry of the melt, e.g., one or more of the impurities of C, N, O, S, P, and by refining grain microstructure by using low hot rolling temperatures, e.g., 950.degree.-1100.degree. C. Chemistry control during refining generally has produced improved ridging characteristics for ferritic stainless steels because of the formation of a second phase, i.e., austenite and martensite. However, formation of this second phase generally reduces the elongation and welding performance of the final products. Temperature control during hot rolling has resulted in operational difficulties as well because of low productivity since this requires a high power hot rolling mill and the hot rolling must be followed by cold rolling in at least two stages with an intermediate anneal between the two cold rollings.
Others have attempted to eliminate ridging by modifying an alloy composition of ferritic stainless steel by the addition of one or more stabilizing elements. For example, U.S. Pat. No. 4,465,525 relates to a ferritic stainless steel having excellent formability and improved surface quality. This patent discloses that boron in amounts of 2-30 ppm and at least 0.005% aluminum can increase the elongation and the r.sub.m value as well as decrease the ridging characteristic. U.S. Pat. No. 4,515,644 relates to a deep drawing ferritic stainless steel having improved ridging quality. This patent discloses that an addition of aluminum, boron, titanium, niobium, zirconium and vanadium all can increase the ferritic stainless steel's elongation, increase the r.sub.m value and enhance the anti-ridging property. More specifically, this patent discloses a ferritic stainless steel having at least 0.01% Al has improved anti-ridging characteristics. U.S. Pat. No. 4,964,926 relates to weldable dual stabilized ferritic stainless steel having improved surface quality. This patent discloses it was known that roping characteristics could be improved by adding niobium alone or niobium and copper to a ferritic stainless steel. However, the addition of niobium alone caused weld cracking. U.S. Pat. No. 4,964,926 discloses that an addition of at least 0.05% titanium to a niobium stabilized steel, i.e., dual stabilized, eliminates weld cracking. U.S. Pat. No. 5,662,864 relates to producing a ferritic stainless steel having good ridging characteristics when Ti, C+N and N/C are carefully controlled. This patent teaches ridging can be improved due to formation of carbonitrides by adding Ti in response to the C+N content in a melt. The steel melt contains .ltoreq.0.01% C, .ltoreq.1.0% Mn, .ltoreq.1.0% Si, 9-50% Cr, .ltoreq.0.07% Al, 0.006.ltoreq.C+N.ltoreq.0.025%, N/C .ltoreq.2.07, (Ti-2S-3O)/(C+N).gtoreq.4 and TixN.ltoreq.30.times.10.sup.-4. U.S. Pat. No. 5,505,797 relates to producing a ferritic stainless steel having reduced intra-face anisotropy and an excellent r.sub.m. This patent teaches good ridging characteristics are obtained when the steel melt contains 0.0010-0.080% C, 0.10-1.50% Mn, 0.10-0.80% Si, 14-19% Cr and two or more of 0.010-0.20% Al, 0.050-0.30% Nb, 0.050-0.30% Ti and 0.050-0.30% Zr. The steel is cast into a slab and hot rolled to a sheet having thickness of 4 mm, annealed, pickled, cold rolled and finish annealed. The slab was heated to 1200.degree. C. and subjected to at least one rough hot rolling pass at a temperature between 970.degree.-1150.degree. C. The friction between the hot mill rolls and the hot rolled steel was 0.3 or less, the rolling reduction ratio was between 40-75% and the hot rolling finishing temperature was 600.degree.-950.degree. C. The hot rolled steel was annealed at a temperature of 850.degree. C. for 4 hours, was cold reduced 82.5% and finish annealed at a temperature of 860.degree. C. for 60 seconds.
As evidenced by the seemingly endless struggle of others, there remains a long felt need for an annealed ferritic chromium alloyed steel that is essentially free of ridging and having excellent deep formability characteristics such has a good r.sub.m value, a high tensile elongation and an annealed uniform grain structure. There remains a further need for an excellent deep formability ferritic stainless steel having good ridging characteristics that does not require a hot processed sheet to be annealed prior to cold reduction. There remains a further need for an excellent deep formability ferritic stainless steel having good ridging characteristics formed from a hot processed sheet that does not have surface defects, i.e., titanium nitride scale and titanium oxide streaks, without requiring surface conditioning of the surfaces of a continuously cast slab prior to hot processing of the slab.